I'm no expert but GNU Ring https://www.ring.cx https://www.ring.cx does look interesting. It doesn't give the federated goodness that Matrix does, but is truly

I'm no expert but GNU Ring https://www.ring.cx https://www.ring.cx does look interesting. It doesn't give the federated goodness that Matrix does, but is truly distributed P2P using DHT. It was absorbed by GNU in 2016, so it does have that going for it.